An ordinary high energy ball milling method for preparing alloy powder, as currently one of the most commonly used technologies for preparing and mechanically alloying nanomaterials and micron materials, generally refines the metal or alloy powder to a nanometer and micron scale by rotation or vibration with a high energy ball mill, that is, putting two or more kinds of powder at the same time into a ball milling tank of the high energy ball mill, and subjecting the powder particles to a repeated process of rolling, pressing, crushing and re-pressing (i.e., repeated cold welding-crushing-cold welding) to make the powder grain constantly refined and the particle size constantly reduced, thus finally obtaining the nanometer and micron ultrafine alloy powder with uniformly distributed structure and composition. Usually the high energy ball mill is used to process the powder simply through rotation or vibration of the ball milling tank, i.e., using the mechanical energy of the milling ball in the ball milling tank, that is, only the mechanical stress field works. However, the current mechanical alloying application is mainly concentrated in the planetary and agitating ball mills, having large energy consumption, low efficiency and other shortcomings.
A plasma generator typically applies a high-frequency electric field to the reaction gas environment under a negative pressure (vacuum), with the gas ionized under the excitation of a high-frequency electric field to produce plasma. These ions have high activity and energy that is sufficient to destroy almost all of the chemical bonds and cause chemical reactions on any exposed material surface, resulting in changes in the structure, composition and groups of the material surface to produce a surface that meets the actual requirements. In addition, the plasma has a fast reaction speed and high processing efficiency, with the modification only occurring on the material surface and having no effect on the performance of the material inside, and is thus an ideal means of surface modification. Plasma surface modification has been widely used in film-like, bulk, granular and other materials, and materials of different shapes have to be subjected to different plasma treatments; for example, for film-like materials (including films, fabrics, non-woven fabrics, wire meshes, etc.), since they can be packed in rolls, they can be processed in a roll-to-roll batch; bulk materials can be placed one by one, and so they are suitable for multi-layer plate electrode processing. However, plasma is seldom used for processing powder particles, and especially the introduction of plasma into the high energy ball milling device is more difficult, which is mainly due to the following two aspects: first, due to the accumulation of powder and the agglomeration among particles, the surface of the particles without exposure to the plasma atmosphere cannot be processed, and it is difficult to get all the particles processed, resulting in incomplete and nonuniform particle processing and a poor processing effect; second, the discharge electrode is seriously damaged under the combined action of the high-speed collision and the high-voltage discharge of the grinding ball in the high energy ball milling tank, thus having a very short life in the ball milling tank. Therefore, there is an urgent need for a plasma assisted high energy ball milling device for processing powder materials.
A patent CN 1718282 A, disclosing a plasma assisted high energy ball milling method, mainly introduced how to achieve and improve the plasma discharge assisted ball milling effect on the basis of an ordinary ball mill, but did not further disclose the specific structure of a main engine of the ball mill or the structure design of the discharge ball milling tank, in particular the material selection and design of the dielectric barrier discharge electrode bar. In fact, the plasma assisted high energy ball mill has various technical problems with the external plasma power supply, the discharge ball milling tank, the dielectric barrier discharge electrode bar and the like, and especially has mutual fitting, local high intensity breakdown discharge, plasma discharging current strength control and other issues in the process of introducing the electrode bar into the ball milling tank, and the electrode bar itself is limited by the various problems affecting life that are caused by the material and structure, which are not resolved by the above invention patent.
Patents CN 101239334 A and CN 1011239336 A respectively disclosed a plasma assisted high energy roller ball milling device and a plasma assisted agitating ball milling device, which were primarily obtained by modification on the conventional roller and agitating ball mill; however, these two ball mills have small mechanical energy and low ball milling efficiency, not only difficult to regulate the ball milling energy in a wide range, but also unsuitable for the plasma assisted high efficient refining effect. The vibrating ball milling device can regulate the ball milling energy in a wide range simultaneously from both the amplitude of the excitation block and the speed of the ball mill.
A patent CN 101239335 A disclosed a plasma assisted high energy planetary ball milling device, which improves the ball milling efficiency of the planetary ball mill based on the traditional planetary ball mill by introducing an electrode bar with an external plasma power supply into the planetary ball milling tank. However, since the planetary ball mill has to achieve rotation and revolution of the ball milling tank, the electrode introduced into the ball milling tank is extremely unstable; in addition, the electrode bar installed in the ball milling tank has a serious hindrance to the collision of the grinding ball, which weakens the ball milling advantage of the planetary structure.
Patents CN 102500451 A and CN 202398398 U disclosed an assisted ball milling dielectric barrier discharge electrode bar, which was provided on the tubular conductive electrode layer with a tubular polytetrafluoroethylene barrier dielectric layer, removing the thread fitting between the two tubes; and this electrode bar could only be applied to a ball milling tank provided at both ends with a through hole. In the actual processing and assembly process, this fitting can never avoid the damage done by the residual air to the electrode bar in the discharge process, and thus the actual life of the electrode bar cannot be greatly improved.
U.S. Pat. Nos. 6,126,097 and 6,334,583 disclosed a planetary high energy ball milling device and a method for preparing nanometer powders, and introduced the structure of an ordinary planetary ball mill and its application in the preparation of nanometer powders. However, these invention patents are limited to the field of the planetary ball mill, and do not involve the application of the external plasma electric field.
Contents of the Invention
For overcoming the drawbacks of mechanical alloying including large energy consumption, low efficiency and heavy pollution, the object of the present invention is to introduce a dielectric barrier discharge electrode bar into a high-speed vibrating ball milling tank with the dielectric barrier discharge (DBD) as a notable and unique discharge approach for generating a plasma, which requires that, on one hand, a solid insulation medium on the outer layer of the electrode bar can simultaneously bear high-voltage discharge and mechanical shock failure of the grinding ball, and on the other hand, the high-speed vibrating ball milling device can uniformly process the powder, thus providing a new type of high energy ball milling device that can efficiently improve the mechanical alloying efficiency of materials and the application method thereof for preparing cemented carbide, lithium ion batteries and hydrogen storage alloy powder materials. Based on the ordinary ball milling technology, with another kind of effective energy inputted to the processed powder by introducing discharge plasmas, the present invention accelerates refinement of the powder to be processed and promotes the alloying process under the combined action of the mechanical stress effect and the external electric field discharge for producing the plasma, thereby greatly improving the processing efficiency and the effect of the ball mill.
The present invention provides an application method for producing cold field plasma discharge assisted high energy ball milled powder, which comprises: first inputting different voltage and current to a discharge ball milling tank of a plasma assisted high energy ball milling device by using an external cold field plasma power supply, then regulating the internal atmosphere (type and pressure of a gas) of the ball milling tank through a controllable atmosphere system, and then making a discharge electrode bar in the discharge ball milling tank produce a corona or glow discharge phenomenon with controllable strength, thus realizing a plasma field high energy ball milling and assisted mechanical alloying process for the processed powder in the discharge ball milling tank.
The present invention also provides a plasma assisted high energy ball milling device using the method for the cold field plasma high energy ball milled powder, which comprises six components, i.e., a vibrating high energy ball milling main engine, an external cold field plasma power supply, a discharge ball milling tank, a discharge electrode bar, a controllable atmosphere system and a cooling system, with the vibrating high energy ball milling main engine being in the form of a vibrating mill;
the discharge ball milling tank comprises a connecting cylinder, a front cover, a rear cover, and a plasma power supply negative grounding electrode connected to the discharge ball milling tank; and
the discharge electrode bar, in the form of a cylindrical rod, is composed of an inner conductive core made of iron (copper) and an outer insulation layer made of polytetrafluoroethylene; the inner conductive core, as an electrode for plasma discharge, is connected to a plasma power supply positive high-voltage electrode, and the outer insulation layer is present as a discharge dielectric barrier layer.
The plasma assisted high energy ball milling device according to the present invention is also characterized in that:
the vibrating high energy ball milling main engine is alternatively in the form of an eccentric vibrating mill;
the external cold field plasma power supply 2 converts a mains supply current into a high-frequency current by using a high-voltage AC power supply in a conversion mode of AC-DC-AC, wherein an FM control mode is used for the DC-AC conversion, the working frequency is adjustable in the range of 1-20 kHz, and the power supply output voltage is in the range of 1-30 kV; the outer insulation layer of the cylindrical rod-shaped discharge electrode bar is alternatively made of a high purity alumina ceramic material;
a tightening end of the conductive core made of iron (copper) in the discharge electrode bar threadedly fits in with the outer insulation layer made of polytetrafluoroethylene, a discharge end fits in with the outer insulation layer by having a bare rod structure, a fitting gap between the conductive core and the outer insulation layer is filled with a heat-resistant adhesive, and the top of the conductive core fits in with a medium in the outer insulation layer by having a spherical structure;
the outer insulation layer made of a high purity alumina ceramic material, composing the discharge electrode bar together with the inner conductive core made of iron (copper), is formed by a direct deposition method or a micro-arc oxidation method;
the discharge electrode bar of the outer insulation layer made of a high purity alumina ceramic material is alternatively covered with a metal sleeve with meshes;
the controllable atmosphere system, mounted above inlet and outlet holes of the discharge ball milling tank, can independently regulate ball milling effects of the plasma on the processed powder under different atmospheric pressure and in various atmospheres of argon, nitrogen, ammonia, hydrogen and oxygen; flanges on both ends of the cylinder of the discharge ball milling tank are sealedly connected to the front cover and the rear cover through a sealing ring and a bolt, respectively, with a through hole and a blind hole for fixing the discharge electrode bar provided in a central position of the front cover and the rear cover, respectively;
a stainless steel sleeve and a rubber sealing ring are embedded in the through hole of the front cover of the discharge ball milling tank, and a stainless steel sleeve gasket is embedded in the blind hole of the inner side of the rear cover; and
the front cover of the discharge ball milling tank is provided on its outer end face with a vacuum valve.
For the application method for producing cold field plasma discharge assisted high energy ball milled powder according to the present invention, with the dielectric barrier discharge providing the plasma, a medium is covered on an electrode placed in the discharge space. When a sufficiently high AC voltage is applied to the discharge electrode, dielectric barrier discharge is generated to break the gas between the electrodes, or a very uniform, scattered, stable and seemingly low pressure glow discharge is formed, thus constituting a unique discharge form with a large number of fine fast pulse discharge channels. For introducing a dielectric barrier discharge electrode bar into the high-speed vibrating ball milling tank, it is required that, on one hand, a solid insulation medium on the outer layer of the electrode bar can simultaneously bear high-voltage discharge and mechanical shock failure of the grinding ball, and on the other hand, the high-speed vibrating ball milling device can uniformly process the powder, thus providing a new type of high energy ball milling device that can efficiently improve the mechanical alloying efficiency of materials and the application method thereof for preparing cemented carbide, lithium ion batteries and hydrogen storage alloy powder materials. Based on the ordinary ball milling technology, the discharge space pressure is set to a non-thermal equilibrium discharge state with a pressure of about 102 to 106 Pa, and discharge plasmas are introduced to input another kind of effective energy to the processed powder, so as to accelerate refinement of the powder to be processed and promote the alloying process under the combined action of the mechanical stress effect and the external discharge plasma, thereby greatly improving the processing efficiency and the effect of the ball mill.
With the following unique advantages of the dielectric barrier discharge plasma of the present invention, the dielectric barrier discharge plasma is clearly a better choice for the introduction of plasma into the high energy ball mill:
First, the dielectric barrier discharge plasma can be generated at atmospheric pressure, which meets the condition that the ball milling needs to be carried out in a protective atmosphere of a certain pressure;
second, since the dielectric layer suppresses the infinite enhancement of micro discharge, the dielectric barrier discharge will not be converted into spark discharge or arc discharge, which ensures that the plasma is not a thermal plasma having strong destructive power on materials, thereby avoiding burning of the ball milling system;
third, the dielectric barrier discharge can be spread evenly on the surface of the dielectric layer, so that the ball milled powder can evenly receive the action of the dielectric barrier discharge plasma; and
finally, under certain conditions, the dielectric barrier discharge can produce quasi-glow or glow discharge, so that it is possible to achieve efficient ball milling in the reaction atmosphere, so as to accelerate refinement of the powder to be processed and promote the alloying process under the combined action of the mechanical stress effect and the external discharge plasma, thereby greatly improving the processing efficiency and the effect of the ball mill.
In the figures: 1. A vibrating high energy ball milling main engine; 2. an external cold field plasma power supply; 3. a discharge ball milling tank; 4. a discharge electrode bar; 5. a controllable atmosphere system; 6. a cooling system; 7. a grinding ball; 31. a cylinder; 32. a front cover; 33. a rear cover; 34. a plasma power supply grounding electrode; 35. a plasma power supply high-voltage electrode; 36. inlet and outlet holes of the tank; 41. a conductive core; 42. an outer insulation layer; 311. a flange; 312. a sealing ring; 313. a bolt; 321. a through hole; 322. a stainless steel sleeve; 323. a rubber sealing ring; 324. a vacuum valve; 325. a polytetrafluoroethylene plate; 326. a ceramic plate; 331. a blind hole; 332. a stainless steel sleeve gasket; 333. a polytetrafluoroethylene plate; 334. a ceramic plate; 411. a tightening end; 412. a discharge end; 413. a spherical structure; 421. a metal sleeve; 51. a pressure reducing valve; 52. a flowmeter; 56. an unloading valve; 541. a ball valve; 542. a ball valve; 551. a filter; 552. a filter; 571. a metal hose; and 572. a metal hose.